Astronomers Discover Terzan 5's True Nature

Observations of a distant cluster of stars in our galaxy have defined a new class of objects that turn out to be galactic building blocks. Researchers used Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST) to study the stars that make up Terzan 5, long though to be a globular cluster. What they found puts that cluster into a very rare class of objects that helped form the Milky Way's core region.

“Webb’s new near-infrared observations, cross-referenced with Hubble’s archival observations, have given us a much clearer picture of the history of Terzan 5,” said Giorgia Zullo, who led the research team studying this oddball cluster. Zullo and her colleagues looked at HST data and images to define the stars that exist in the cluster.

Terzan 5 is most likely the remnant of a much more massive stellar system that initially formed 12.5 billion years ago, when the Milky Way was still coalescing. Somehow it survived the various mergers and never really mixed with the other stars in the bulge. “For some reason, this peculiar clump of stars formed separately from the bulge and was not destroyed as the bulge itself formed,” said Francesco R. Ferraro, a professor at the University of Bologna and principal investigator of the Webb observations. “Terzan 5 is what we now call a bulge fossil fragment because it resembles the primordial clumps that contributed to the formation of the bulge.”

A Hubble Space Telescope view of Terzan 5 was the first show individual stars in this cluster. The ages of its stars span nearly the entire age of the galaxy. It has the most millisecond pulsars of any such cluster. Courtesy NASA/ESA/STScI.

Even more interesting is that Terzan 5 has four different generations of stars, which is not something globular clusters have. Terzan 5's populations of stars indicate that it went through four distinct eras of star formation. It's more closely related to clumps of stars that existed in the Milky Way's earliest epochs. These clumps and their stars spread out and merged to form our galaxy's central bulge. Terzan 5 didn't and has, instead, gone on to form new generations of stars and retain its globular shape.

The Milky Way's globular clusters are a tightly woven part of our galaxy's history. Today we see them swarming around the central bulge. Most have have similar ages (dating back around 10-12 billion years), with stars of similar metallicity (that is, chemical elements heavier than hydrogen and helium). Many of those stars are "metal poor", unlike the Sun, which has a more enriched chemical makeup. The ages of stars in g